1. Field of the Invention
The present invention concerns position encoders and in particular absolute encoders employing a "linear" light source.
2. Background Art
Position encoders ("encoders") are electro-mechanical devices producing a digital output related to the position of a movable element of the encoder. In one such encoder design: a rotary encoder, the movable element is a rotatable shaft attached to a disk shaped rotor. The rotor has an optically readable pattern marked on its surface, formed by alternating opaque and transmissive frames.
The frames are illuminated from one side by an illumination source, light traveling from the illumination source through the opaque and transmissive frames of the rotor and then through similar frames in a stationary stator, to be detected by a stationary photodetector. Rotation of the shaft moves the rotor which in turn causes a fluctuation in the light transmitted through the rotor and stator thus producing a signal that may be decoded into a digital indication of shaft movement.
Encoders may be either absolute encoders or incremental encoders. Incremental encoders provide only an indication of the change in position of the encoder shaft. The rotor of an incremental encoder ordinarily contains a uniform periodic pattern whose movement past a photodetector creates an index signal indicative of the amount that the shaft has rotated. Two or more photodetectors arranged with an offset of 90.degree. ("quadrature") may be used to provide an indication of the direction of rotation as well as amount of rotation of the shaft, as is understood in the art.
Absolute encoders, on the other hand, produce a unique digital code word for each encoder position. The rotor of an absolute encoder may carry a series of concentric tracks whose opaque and transmissive segments, examined along a line of radius, reveal a binary or Grey code value indicative of shaft position. Each track provides the value of one bit and is read by a separate photodetector to produce an output digital word.
One difference between the construction of an absolute and incremental encoder is that, in an incremental encoder, the light source may illuminate a relatively wide area of the rotor (because the pattern on the stator and rotor is spatially periodic on a small scale). Light is gathered over a series of adjacent frames separated by one of the periodicity to increase the total light signal. The stator mask and the photodetector effectively "average" the light transmitted by many frames within this illuminated area.
In contrast, in an absolute encoder, the mask and rotor patterns are not periodic. Any light passing through the rotor or stator outside of the aligned frames degrades the signal produced by the aligned frames reducing the contrast in signal strength produced by transmissive and opaque frames.
For this reason the light source in an absolute encoder must be masked to a narrow beam one frame wide. The light must also be highly collimated so that the detector receives the masked light primarily along an axis defined by the aligned frames of the stator and rotor. Collimation generally defines the degree to which the wavefronts of the propagating light are planar, whereas the masking aperture defines the width of the beam of propagating light.
One problem with the very narrow beams used in absolute encoders is that little light energy is transmitted to each photodetector. Typically, therefore, the attenuation introduced by the slit mask forming the narrow beam is compensated for by first focusing the light source on the mask. If the light source is linear, i.e., an incandescent bulb with a linear filament or a linear array of light emitting diodes, this focused image will also be similarly linear, reducing somewhat the energy lost in the masking process. Even so, such masking systems are low in efficiency and generate unnecessary heat and consume unnecessary power.
As the resolution of the absolute encoder increases, the degree to which the beam is collimated also becomes increasingly important. Poor collimation may undo the intended effect of the mask and cause light leakage between frames of the mask pattern degrading the encoder signal. Precise collimation in the above described focussing and masking systems depends on accurate alignment of the focusing elements which generate the masked beam. Such precise alignment makes the manufacture of such encoders more expensive and renders the encoders susceptible to physical shock which may misalign the optical elements.